function preOrder(root){
    if(!root){
        return root
    }
    console.log('exec',root.value);
    preOrder(root.left)
    preOrder(root.right)
}
function postOrder(root){
    if(!root){
        return root
    }
    preOrder(root.left)
    preOrder(root.right)
    console.log('exec',root.value);

}